1. Field of the Invention
The present invention is related to a driving method for a bistable display device, and more particularly, to a driving method of the bistable display device for reducing a number of frames required when switching images.
2. Description of the Prior Art
Paper is a commonly-used display medium, due to advantages of wide viewing angle, thin and flexible body, and being easy to carry around. Due to popularized printing technology, a user can easily print out a massive amount of data on paper. However, manufacturing paper consumes substantial natural resources, and information on conventional paper is usually not updatable, or can only be updated a limited number of times. Paper-like display devices are gaining popularity, since paper-like display devices possess both the advantages of paper and the updatable property of electronic devices.
Paper-like display devices can be realized with bistable display devices. A bistable display device only consumes power when changing displayed images, and the displayed images can be displayed without applying external voltages. Bistable display devices can be categorized as electrophoretic display devices and cholesteric liquid crystal display devices. A light valve layer of an electrophoretic display device or a cholesteric liquid crystal display device is capable of being in a first state or a second state. For instance, the first state is a bright state and the second state is a dark state.
Taking the electrophoretic display device as an example, in the first state, the light valve layer displays a white substance, and the white substance reflects light to display the bright state. In the second state of the electrophoretic display device, the light valve layer displays a black substance or a colored substance, and the black substance absorbs light to display the dark state, or the colored substance absorbs light to display a colored state.
The cholesteric liquid crystal display device comprises characteristics of bistabiility, high contrast and high color saturation. The cholesteric liquid crystal display device only consumes power when changing displayed images, and the same displayed image can be displayed without applying external voltages. Characteristics of a cholesteric liquid crystal make the cholesteric liquid crystal suitable for reflective display devices. Hence, a reflective cholesteric liquid crystal display device excels in power consumption when displaying still images.
A distinct behavior of the bistable display device, taking the cholesteric liquid crystal display device as an example, is that the light valve layer is stably in either a planar state or a focal conic state. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a light valve layer CLCL of a cholesteric liquid crystal display device. As shown in FIG. 1, a second substrate S2 is disposed below a first substrate S1, and the light valve layer CLCL is disposed in between the first substrate S1 and the second substrate S2. The second substrate S2 is disposed in between the light valve layer CLCL and a light absorption layer LAL, and the light absorption layer LAL is disposed below the second substrate S2. The light valve layer CLCL comprises a plurality of liquid crystals CLC. The light L passes through the light valve layer CLCL via the first substrate S1, and is then absorbed by the light absorption layer LAL through the second substrate S2. Amount of the light L reflected by the liquid crystals CLC corresponds to arrangement of the liquid crystals CLC, and affects how much light L is absorbed by the light absorption layer LAL. In the planar state, the liquid crystals CLC in the light valve layer CLCL are aligned, which corresponds to highest reflectivity. In the focal conic state, the liquid crystals CLC in the light valve layer CLCL are arranged orderlessly, which scatters the injected light L. Compared to the planar state, the focal conic state corresponds to a relatively lower reflectivity. Generally, the light valve layer CLCL displays the first state (e.g. the bright state) in the planar state, and displays the second state (e.g. the dark state) in the focal conic state. In addition, the light valve layer CLCL can also be in a transient state, which is the homeotropic state. In the homeotropic state, the liquid crystals CLC in the light valve layer CLCL are aligned vertically (parallel to the externally applied electrical field), so almost all of the light L can pass through the light valve layer CLCL and be absorbed by the light absorption layer LAL.
The state of the light valve layer of the cholesteric liquid crystal display device can be altered according to an electrical field applied to the light valve layer. Please refer to FIG. 2. FIG. 2 is a diagram illustrating changing the state of the light valve layer according to different electrical fields applied. In FIG. 2, an increase of the electrical field applied to the light valve layer is represented by “+”, and a decrease of the electrical field applied to the light valve layer is represented by “−”. As shown in FIG. 2, the light valve layer can be transformed to the focal conic state from the planar state by applying a relatively smaller electrical field (e.g. applying a voltage of approximately 5-20 volts). The light valve layer can be transformed to the homeotropic state from the planar state or the focal conic state by applying a relatively larger electrical field (e.g. applying a voltage approximately higher than 40 volts). If the applied electrical field is removed swiftly (e.g. applying a voltage of approximately 0-5 volts) when the light valve layer is in the homeotropic state, the light valve layer is transformed back to the planar state. If the applied electrical field is removed slowly when the light valve layer is in the homeotropic state, the light valve layer is transformed to the focal conic state. The light valve layer in the focal conic state can also transform to the focal conic state of an even lower reflectivity by applying the electrical field.
However, the light valve layer in the focal conic state cannot transform back to the planar state directly. The relatively larger electrical field must be applied to the light valve layer first to transform the light valve layer to the homeotropic state, then the applied electrical field is removed quickly for the light valve layer to transform back to the planar state. Further, if the light valve layer is to transform from a focal conic state of a lower reflectivity to a focal conic state of a higher reflectivity, the light valve layer must transform back to the planar state first through the homeotropic state, and then transform to the focal conic state of the higher reflectivity by applying an electrical field of corresponding magnitude.
In other words, for the reflective bistable display device, an image of high gray scale can be switched to low gray scale by applying voltages directly. However, for switching the image of low gray scale to high gray scale, the light valve layer must be reset back to the planar state first through the homeotropic state, and then a corresponding voltage is applied for the light valve layer to display the target gray scale from the planar state.
Therefore, reset must be performed by the reflective bistable display device when switching displayed images. Taking the cholesteric liquid crystal display device as an example, reset is performed by applying the relatively larger electrical field for the light valve layer to transform to the homeotropic state, then quickly removing the electrical field for the light valve layer to transform back to the planar state, so that a pixel can change from displaying a higher gray scale to a lower gray scale. Hence, when utilizing the conventional method to drive the bistable display device to play back videos or motion graphics, a higher number of frames is required to display each image. Consequently, a higher frame rate is required for the display panel.